In an impact printer of the type in which a hammer impacts the character to be printed, there is typically a drive coil for driving the hammer into the character and paper, and a spring for returning the hammer to the backstop. The return velocity will be high immediately after impact, and the hammer will continue toward the backstop because of the spring bias, but it will slow down as it travels toward the backstop because of friction and windage losses. The maximum printing speed is limited by the time required for the hammer to return to rest at the backstop. If the hammer still has any residual energy, in the form of a velocity in either direction, or in the form of a position anywhere other than in contact with the backstop, the energy applied to the next character will be added to the residual energy in the hammer, resulting in a hammer impact which is too large or small. Of course, this results in a loss of print quality. If the hammer velocity could be brought smoothly to zero at the backstop, there would also be a reduction in the noise level, which would make the printer less objectionable in an office environment.
Various methods can be used to control the return velocity to bring it to zero at the backstop. For instance, a velocity or position sensor can be used to measure or calculate the position and velocity of the hammer, and an analog servo system can be used to control the velocity. However, the cost of such a system probably would be excessive for a small, commercial printer.
Another method would be to apply a drive pulse of predetermined timing and duration during the return of the hammer to result in zero velocity as the hammer just reaches the backstop. Since large letters like M and W require a greater velocity to achieve a greater printing force, and since the coefficient of restitution is close to one at the impact point, there are a number of likely return velocities, each having its own damping pulse parameters. The problem with this system is that the predetermined pulse parameters can only be determined for an average case, and cannot take into accound variations in manufacturing tolerances, temperature effects, effects of wear on the mechanical components, etc.